@Rock and Roll Ken Doll pssst but what if LW is Junot Diaz as suggested above DON'T LET HIM KNOW WE'RE ON TO HIM.
@Emmanuelle Cunt his first girlfriend who then became his wife and then six years later became his ex-wife, and then he and I met. I...have dated many more people much less seriously than that! Not that that's a bad thing.
Let me save you some of my own suffering by advising that whoever you date after this guy from now until forever, you canNOT tell him about Best/Worst Guy. He won't understand. He will probably be upset with you. I've spent the past two years trying to make my boyfriend understand that the man I dated when I was eighteen does not necessarily reflect on my true morality/best self/current feelings about marital fidelity.
LW#3: start dating one of your dad's male friends who's around his age.
LW#1: I was in a situation like this (along with twenty other commentators?) and we eventually broke up for different long-distance-commitment-related reasons, but I'm sorry you're going through this difficulty. In some ways, it's easier to be friends with someone in that situation than to be his girlfriend. As his girlfriend, I always thought the depressed periods somehow reflected badly on me. As his friend, I just want to help him through them in any way I can. I hope that you and he both have friends you can rely on when you need someone outside the relationship to listen to you, or reassure you, or show you different ways of being.